In some computers systems, the need may arise to disable the computer when an input clock signal exceeds an upper limit. For instance, using a clock that is too fast may cause problems in a system, since some of the components in the system may not be able to operate at the higher frequency.
In the fabrication of microprocessors, it is common to find out that some of the microprocessors are able to operate at higher frequencies than others even though all are fabricated with the same processor. Process variations affecting, for example, the thickness of oxides can cause some wafer runs to yield parts that operate at much higher frequencies than in other wafer runs. Some companies test the upper frequency performance of newly fabricated microprocessors and sort them into "frequency" bins. Consequently, some microprocessors when sold are graded (and labeled) for higher frequencies than others. The microprocessor may not function properly for frequencies above its labeled frequency, especially under stressful ambient conditions or marginal power supply potential.
Companies have found that parts labeled with one frequency are sometimes relabeled after being sold by the manufacturer with higher frequencies, and then resold. These relabeled parts could end up in computer systems sold to operate at higher frequencies than the manufacturer intended. An innocent purchaser of such a computer could find that the computer operates well part of the time but fails at other times. The failures may be difficult to trace, and if traced to the microprocessor, reflect badly on the manufacturer.
At least one manufacturer has incorporated a speed governor in its microprocessors which cause the microprocessors to fail if the clocking signals applied to them are higher than the frequency they were originally sold for. When this is done, the microprocessor could never be used in higher frequency applications since it will never operate at the higher frequencies.
In U.S. Pat. No. 5,107,523, entitled "Processor Clock Governor", issued Apr. 21, 1992 and assigned to the assignee to the present invention, a circuit is described for disabling a microprocessor when a clock with an improper frequency is used. A problem with this circuit is that the manner in which the "kill" signal is generated is itself subject to process variations and hence may not be settable with the precision desired.
In U.S. Pat. Nos. 5,280,605 and 5,274,337, a clock speed limiter is disclosed and uses an integrated resistor as the basis for a timing reference. The resistance value is sensitive to temperature variation, which makes the timing reference variable with temperature. In other words, because the resistance value is sensitive to temperature variation, the cut-off frequency is not constant with temperature, making it difficult to set a reliable cut-off value without losing yield.
As will be disclosed, the present invention provides a circuit that is substantially independent of temperature and process variation and additionally provides other advantages.